linux-kernel - Re: [PATCH] sched/fair: Simplify some logic in update_sd_pick

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Date: Fri, 2 Feb 2024 11:07:12 -0600
From: David Vernet <void@...ifault.com>
To: Valentin Schneider <vschneid@...hat.com>
Cc: linux-kernel@...r.kernel.org, mingo@...hat.com, peterz@...radead.org,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, bristot@...hat.com, kernel-team@...a.com
Subject: Re: [PATCH] sched/fair: Simplify some logic in
 update_sd_pick_busiest()

On Fri, Feb 02, 2024 at 06:01:22PM +0100, Valentin Schneider wrote:
> On 02/02/24 01:02, David Vernet wrote:
> > When comparing the current struct sched_group with the yet-busiest
> > domain in update_sd_pick_busiest(), if the two groups have the same
> > group type, we're currently doing a bit of unnecessary work for any
> > group >= group_misfit_task. We're comparing the two groups, and then
> > returning only if false (the group in question is not the busiest).
> > Othewise, we break, do an extra unnecessary conditional check that's
> > vacuously false for any group type > group_fully_busy, and then always
> > return true.
> >
> > Let's just return directly in the switch statement instead. This doesn't
> > change the size of vmlinux with llvm 17 (not surprising given that all
> > of this is inlined in load_balance()), but it does shrink load_balance()
> > by 88 bytes on x86. Given that it also improves readability, this seems
> > worth doing.
> >
> > As a bonus, remove an unnecessary goto in update_sd_lb_stats().
> >
> 
> Given that's a different scope than what the rest of the patch touches, I'd
> rather see this as a separate patch.
> 
> Other than that:
> Reviewed-by: Valentin Schneider <vschneid@...hat.com>

Thanks, would you like me to send a follow-on series split into two with
your tag on both? Or were you just letting me know for next time?

We could also update this check to only do a strict greater than to
avoid unnecessary writes, but I figured it was preferable to have no
logical changes for this iteration:

return sgs->group_misfit_task_load >= busiest->group_misfit_task_load;

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